Hand feeding works best with two people. One person holds the bird and the other inserts the tube and presses the plunger. Here's how we do it. One person holds the pigeon in their right hand, placing the bird on a counter or other flat surface, and cupping the right hand over the bird's back to hold it down firmly. While holding the bird, place the thumb of the right hand behind the bird's head. This will help to hold the head up and keep the neck straight, making it easier for the other person to insert the tube. With the left hand, grasp the lower half of the bird's beak. This will help open the beak. The other person grasps the top half of the bird's beak, opens the beak, and inserts the tube, being careful to insert it gently and to place it over the tongue into the throat, keeping it against the back of the throat, and gently easing it down into the crop (see diagram below). If you feel any resistance, remove it and try again. For a good sized squeaker, the tube will go in farther than you think it will. Remember, if you're using a soft rubber tube, you can't hurt him with it. When you've got it in, push the syringe plunger. If everything went perfectly, the crop will instantly fill, and you can withdraw the tube with no muss and no fuss. You actually get quite quick at this after doing it a few times. We've found that the person using the syringe sometimes has to lay the syringe on the counter, insert the tube, then pick up the syringe and press the plunger. It's too hard sometimes to do it all with the syringe in your hand--so have a long enough piece of tubing to allow you to do this.
Sometimes food will start coming out of the baby's mouth as you're pressing the plunger. This probably means the tubing is not in far enough. Take it back out, let everybody calm down, and try again, inserting the tube farther this time. If you have trouble inserting the tube, make sure you are keeping the bird's head up and its neck as straight as possible.
Be gentle, stay calm, and remember that if you're having to hand feed the bird, it really needs your help and will die without it. We were very clumsy and nervous the first time we had to do this, but we've hand fed several squeakers and also some sick or injured adult pigeons, and we haven't killed one yet.